Only render reflection of an object on an invisible object
A bit late, but this can be done with two render layers.

Put the objects that are to be reflected on one layer and the mirror on another layer, and make sure that the camera and any light sources are visible on both those layers. Under Relations in the Object tab, make sure both layers are selected (hold Shift while clicking LMB to select more than one).



Go to Render Layers and add one render layer. Under Scene select the two layers on which your objects are.
On the first render layer, make sure only the layer containing the objects that are to be reflected is selected. Leave everything else unchanged.



On the second render layer, make sure only the layer containing the mirror is selected, and under Passes make sure only Glossy: Indirect is selected.



Now go to the compositor. Make sure you're on the Compositing tab and have enabled Use Nodes. One input and one output node should already be there. Leave those alone for now. Add another one one, Render Layers (ShiftA > I > R or Add > Input > Render Layers) and select the render layer containing your mirror. It should only have a single output socket, Glossy Indirect.

Add a Math node (ShiftA > N > M) and set it to Greater Than. Connect the Glossy Indirect to the upper input socket of the math node. This checks if the intensity of the colour is greater than the value specified in the lower input of the math node. Anything greater than this will be set to 1 and anything equal to or less than this will be set to 0. This output will be a mask, and the value you specify is the threshold at which to mask. Connect that to a Blur node (ShiftA > F > L). I'm using Gaussian and enabled the Bokeh option, but the exact settings are a matter of taste. I found that I liked this best, but you may prefer something different.

Add a Set Alpha node and connect the output from the blur node to the Alpha input and the Glossy Indirect output from the Render Layers node to the Image input.

Add an Alpha Over node, and connect the Image output from the first render layer to the upper input and the output from the Set Alpha to the lower input, the the output from the Alpha Over to the Image input of the Composite node that should already be there. Make sure Use Alpha is enabled.



I used a slight roughness on the glossy shader for the mirror, and this makes the reflections a bit blurry. This will cause some parts of the reflection to have very low intensity, but still greater than 0, which can make it appear as if the mirror is showing although there are no reflections (i.e. black edges around the reflections). The threshold (the value in the Math node) controls how much of these will be visible. The blur node blurs the mask slightly, to get the reflections to blend better with a background image. If your mirror's roughness is set to 0.000, you may get good results with a threshold of 0 and no blur node.

Since this uses the fact that reflections are indirect light, it will not work as expected if the objects whose reflections you want to see emit light. And using Glossy: Direct instead, will also make the reflections of the environment visible. Remedying that will be a bit more complicated. If needed, I'll look into that too, but for now, this is what I have.

The result...



And the .blend I've used in this example.



The PBR Dielectric node group used in the included .blend was taken from
How to Make Photorealistic PBR Materials - Part 1 courtesy of Andrew Price.